Hot Surface Igniter With Fuel Assist

ABSTRACT

A large-scale combustion system has a fuel line having a burner end, a burner at the burner end of the fuel line, a source of fuel flowing through the fuel line to the burner, a hot surface igniter adjacent to the fuel line, a flame holder disposed adjacent to the hot surface igniter, and a source of igniter combustion air. The hot surface igniter is activated to ignite the source of fuel, the ignited fuel leaving the burner, and the hot surface igniter is protected from a heat caused by the ignited fuel. A flame detector detects ignition of the source of fuel and in response to detection of ignition the hot surface igniter is deactivated.

FIELD OF THE INVENTION

The present teachings relate generally to large-scale combustion systems and, more particularly, to hot surface ignition devices in such systems.

BACKGROUND OF THE INVENTION

Large-scale combustion systems (e.g., utility & industrial boilers) use controlled combustion of fuel to provide a source of heat. A burner mixes fuel and oxygen together and, with the assistance of an ignition device, provides a platform for combustion. Controls may regulate the ignition device, fuel supply, air supply, and pressure. By ensuring efficient ignition and mixing of air and fuel as it enters the burner, combustion efficiency can be maximized while NOx emissions can be reduced.

When incomplete combustion occurs, the energy of the fuel is not completely released as heat and the combustion efficiency is reduced. Inefficient ignition and improper control of flame stability and flame location, particularly during cold startup, process upsets, or turndown conditions, may result in undesirable combustion performance, higher NOx emissions, and/or unburned fuel. This could lead to substantial pockets of fuel in a furnace and the possibility of an uncontrolled energy release as unburned fuel could ignite and cause an explosion.

Large-scale combustion systems have typically used a form of pilot light as an ignition device. A pilot light (also called an igniter) is a small flame with a separate fuel source used to ignite a more powerful burner. The pilot light may be incorporated into the burner or affixed adjacent to the burner. Since pilot lights utilize separate fuel sources, they add complexity and waste fuel. In some versions, a pilot light is kept alight even when the main burner is not active. However, pilot lights can be blown out by wind and gas leakage. In other versions, electrical igniters have been used to light the pilot light, which is then used to ignite the main burner. However, electrical igniters have a drawback in that the high voltage can be hazardous, and sometimes shorting occurs with flue gas recirculation or with the buildup of carbon or other materials resulting from flames, with the result that no sparking occurs.

Since the high temperature of a burner in a large-scale combustion system can destroy an ignition device, the ignition device must be out of the line of main combustion or moveable out of line of main combustion when not in use. However, moving the ignition device requires additional mechanical components and complexity, which decreases reliability.

Hot surface igniters are another type of ignition device that have been used in cooking ovens and small residential boilers. They are special ignition devices for directly igniting a main fuel source without the need for a pilot fuel. They have fewer mechanical parts and as a result are more reliable. Hot surface igniters are usually made from a material like silicon carbide or nitride. They work like a light bulb filament, and as electricity passes through the hot surface igniter they heat up. Hot surface igniters have traditionally not been used in large-scale combustion systems because such systems produce very high temperatures that would consume the hot surface igniter if placed adjacent to the line of main combustion.

U.S. Pat. No. 7,303,388, the content of which is incorporated by reference in its entirety, discloses “ignition-assisted fuel lances” which utilize a secondary fuel source (e.g., pilot fuel) to introduce a staging fuel (e.g., pilot light flame) into the combustion zone. The '388 patent requires a separate fuel source for its fuel lances. In addition, the '388 patent does not disclose the use hot surface igniters. U.S. Patent Application Publication No. 2012/0282555, the content of which is incorporated by reference in its entirety, discloses a pilot light ignition assembly for igniting waste gases venting from waste gas stacks. In particular, it discloses affixing the assembly to a waste gas stack and using a hot surface igniter to ignite pilot fuel, which is then used to ignite the waste gas.

What is desired is to utilize the benefits of hot surface igniters in large-scale combustion systems, such as those in industrial and utility boilers. What is also desired is to do so without requiring additional mechanical components to retract the hot surface igniter out of the line of combustion. Therefore, it would be beneficial to have a superior system and method for a hot surface igniter with fuel assist.

SUMMARY OF THE INVENTION

The needs set forth herein as well as further and other needs and advantages are addressed by the present embodiments, which illustrate solutions and advantages described below.

In accordance with one aspect of the present invention, a large-scale combustion system has a fuel line having a burner end, a burner at the burner end of the fuel line, a source of fuel flowing through the fuel line to the burner, a hot surface igniter adjacent to the fuel line, a flame holder disposed adjacent to the hot surface igniter, and a source of igniter combustion air. The hot surface igniter is activated to ignite the source of fuel, the ignited fuel leaving the burner, and the hot surface igniter is protected from a heat caused by the ignited fuel.

In some embodiments, the hot surface igniter is protected from the heat by a side stream in the fuel line, the hot surface igniter disposed in the side stream, some of the source of fuel flowing through the side stream and cooling the hot surface igniter when deactivated. In other embodiments, the hot surface igniter is protected from the heat by a retractor retracting the hot surface igniter away from the heat when deactivated.

In some embodiments, the system further includes a flame detector detecting ignition of the source of fuel at the burner, the hot surface igniter deactivated in response to detection of ignition. In some embodiments, the hot surface igniter is deactivated after a predetermined amount of time.

In some embodiments, the system further includes a controller controlling the amount of fuel or air to the burner.

In some embodiments, the source of fuel comprises natural gas. In some embodiments, the source of fuel comprises a liquid fuel.

In some embodiments, the hot surface igniter is activated when no flame is detected at the burner.

In some embodiments, the large-scale combustion system comprises an industrial boiler. In other embodiments, the large-scale combustion system comprises a power generator.

In accordance with another aspect of the present invention, a large-scale combustion system has a fuel line having a burner end, a burner at the burner end of the fuel line, and a source of fuel flowing through the fuel line to the burner. An ignition device is adjacent to the fuel line, the ignition device activated to ignite the source of fuel, the ignited fuel leaving the burner. A side stream is in the fuel line, the ignition device disposed in the side stream, some of the source of fuel flowing through the side stream and cooling the ignition device when deactivated. A flame detector detects ignition of the source of fuel at the burner, the ignition device deactivated in response to detection of ignition.

Other embodiments of the system and method are described in detail below and are also part of the present teachings.

For a better understanding of the present embodiments, together with other and further aspects thereof, reference is made to the accompanying drawings and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an embodiment of a fuel line utilizing a hot surface igniter in accordance with the teachings of the present invention.

FIG. 2 is a schematic diagram showing the fuel line of FIG. 1 where the hot surface igniter is retractable.

FIG. 3 is a schematic diagram showing the fuel line of FIG. 1 where the hot surface igniter in disposed in a side stream.

FIG. 4 is a schematic diagram showing a large-scale combustion system incorporating the fuel line of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The present teachings are described more fully hereinafter with reference to the accompanying drawings, in which the present embodiments are shown. The following description is presented for illustrative purposes only and the present teachings should not be limited to these embodiments.

Large-scale combustion systems include utility power plants, paper & pulp mills, chemical plants, oil refineries, ethanol plants, marine vessels, and diesel engines, among others. Typically, utility & industrial boilers and transportation industries utilize steam generators ranging in size from 10 MW to 800 MW (50,000 to 5,500,000 lb/hr steam flow) and run on any number of different types of fuels, including coal, natural gas, fuel oil, refinery gas, and biofuel, although not limited thereto. Such systems require reliable ignition sources in order to maximize efficiency and to minimize risks associated with unintended explosions.

Typically, large-scale combustion systems utilize pilot lights such as that described in U.S. Pat. No. 7,303,388. However, pilot lights require additional complexity of a separate fuel source in order to keep the pilot light burning. Further, a pilot light can be blown out, requiring re-lighting and the risk that the gases released by the burner do not timely combust.

In order to address some of the failings in the prior art, one embodiment of the present invention is directed to use of a hot surface igniter with “fuel assist.” The hot surface igniter may be placed in a “side stream” of the main fuel source. This provides the benefits of use of a hot surface igniter over prior ignition devices, without the need for additional mechanical components to move the hot surface igniter out of the line of combustion. Instead, the flow of fuel can be used to cool the hot surface igniter when not in use, discussed further below.

There are a number of benefits of hot surface igniters over previous types of ignition devices used in large-scale combustion systems. The hot surface igniter may be more dependable because it has fewer mechanical parts. It typically uses a wire as the igniter, which has enough electrical resistance so that when a current passes through it, it generates a large amount of heat.

A hot surface igniter may be positioned in the path of flow of a fuel or fuel/air mixture to ignite the fuel. As the fuel passes the heated hot surface igniter, the fuel is ignited and the main burner is lit to produce heat. The primary intent is to use the hot surface igniter with the main gas valve open as a Class 3 special igniter. Once the main flame is detected, the power to the hot surface igniter is removed. Hot surface igniters may also utilize a sensor to detect the igniter current flow or glow and open the main burner valve. An electrically operated valve may control the fuel source and be energized to open when the hot surface igniter has heated it to a sufficient ignition temperature. Alternately, just a timer can be used to allow the hot surface igniter to heat up and reach temperature before the main fuel valve is opened.

A very high temperature may be needed to light a main burner, so hot surface igniters may be made out of materials that can both generate and withstand a flash of heat. Silicon carbide is one of the most common materials, a synthesis of silicon and carbon that can quickly become red hot when introduced to an electrical current. Silicone nitride is another common material.

The National Fire Protection Association (NFPA) classifies hot surface igniters as “3.3.85.4 Class 3 Special Igniters” in the Boiler and Combustion Systems Hazards Code. This is a special high energy electrical igniter capable of directly igniting main burner fuel. Hot surface igniters do not require a separate fuel source and cannot be blown out (like pilot lights). Flame detectors, which may comprise flame rods and ultraviolet or infrared scanners, may monitor the flame condition and deactivate the burner in the event of a non ignition or other unsafe condition.

Class 3 (C3) igniters typically range from 4-16 joules power input to directly light a main flame. These have been developed for oil firing originally and are now used for gas firing, although they generally have not been used for coal or other solid fuels. A traditional ignition process involves inserting a spark rod into the fuel spray and igniting the fuel directly. The system charges capacitors that are then discharged with higher energy release then a high voltage transformer. A high voltage transformer often used with C1, 2, 3 igniters may convert 110VAC to about 6-9000VAC, with a continuous spark. These are often 2.4 Joule power input. While hot surface igniters are currently used for ignition of gas burners on dryers, ranges and residential furnaces, large-scale combustion system burners may be in the range of 100 mmBtu/hr.

Referring now to FIG. 1, shown is a schematic diagram showing one embodiment of a fuel line 110 utilizing a hot surface igniter 104. As shown, fuel 114 enters the fuel line 110 past the hot surface igniter 104 to the burner end 108. The hot surface igniter 104 is affixed to the wall 100 of the fuel line 110. In operation, the fuel 114 may ignite as it passes the activated hot surface igniter 104 and combusting fuel 114 may leave the burner end 108 of the fuel line 110. Air 116 is controlled by vents 102 to modulate the mixture of fuel and air.

This embodiment may be appropriate for smaller scale combustion systems where the temperature in the fuel line 110 does not get hot enough to destroy the hot surface igniter 104, For example, this embodiment may be used for pilot lights. However, in large scale combustion systems the temperature caused by the flame at the burner end 108 may be too hot to allow for the presence of the hot surface igniter 104 without any apparatus for protecting or cooling it from heat (discussed further below). In any case, the hot surface igniter is shut off when main flame is detected and good cooling will occur also by the gas passing over the hot surface igniter.

A flame holder 117 is provided downstream of the hot surface igniter 104 in order to stabilize and hold the pilot flame in place, as least long enough for the pilot flame to ignite the main flame. Pilot combustion air 118 is also provided, for example, through vent 119, in order to aid in pilot flame creation and stabilization. As flame holders are well-known in the art in other contexts, the configuration and operation of flame holder 117 is not discussed herein in detail.

Referring now to FIG. 2, shown is a schematic diagram showing one embodiment of a fuel line 110′ utilizing a retractable hot surface igniter 104. A mechanism for protecting the hot surface igniter 104, such as a retractor 105, protects the hot surface igniter 104 from a heat caused by the ignited fuel (may be generically referred to as an igniter guard assembly). Otherwise, excess heat may consume the hot surface igniter 104.

Similar to the embodiment shown in FIG. 1, fuel 114 may ignite as it passes the activated hot surface igniter 104 and leave the burner end 108 of the fuel line 110′. However, in this embodiment a retractor 105 retracts the hot surface igniter 104 out of the path of combustion when not in use.

In some embodiments, particularly with large-scale combustion systems, the combusting fuel may reach temperatures over 1800° F., although not limited thereto. As a result, the hot surface igniter 104 must be protected from the heat in some way. As shown, the retractor 105 repositions the hot surface igniter 104 along a range of motion 120 in the line of the fuel 114 for ignition, and away from the line (or otherwise protected from any heat) when not in use. The retractor may retract the hot surface igniter 104 when ignition is detected or after a predetermined amount of time (e.g., 5 seconds, 10 seconds, etc.), although not limited thereto.

Referring now to FIG. 3, shown is a schematic diagram showing one embodiment of a fuel line 110″ utilizing a hot surface igniter 104 with a side stream 112. Because of the heat caused by combusting fuel 114 leaving the burner end 108 of the fuel line 110″, it is necessary to protect the hot surface igniter 104. However, it may not be preferable to utilize additional components such as a retractor 105 (shown in FIG. 2), which may add complexity and potential for failure. In another embodiment, a direct spark type ignition device may be used, although not limited thereto.

As shown, a side stream 112 comprising a side wall 106 in the fuel line 110″ is provided. No retraction may be needed because the flow of fuel may provide sufficient cooling for protecting the hot surface igniter 104, especially after the hot surface igniter 104 is turned off. The fuel is supplied as a side stream from the main fuel source (also referred to as “fuel assist”). This acts as a cooling medium on the hot surface igniter 104 when the main fuel is on. For example, whereas the heat from the burner may be in excess of 1800° F., natural gas fuel may be only 70° F. When the main fuel is off, the hot surface igniter 104 may be cooled by burner air, although not limited thereto.

Referring now to FIG. 4, shown is a schematic diagram showing one embodiment of a large-scale combustion system 400 according to the present teachings. As shown, the hot surface igniter 104 uses the main fuel 114 source in order to ignite the burner. Similar to FIG. 3, fuel 114 may be diverted to a side stream 112 where it can cool the hot surface igniter 104 when it is not activated.

Air from a windbox may be provided by registers 136 to help regulate the burner. A flame stabilizer 132 near the burner throat 134 may provide a flame to heat boiler tubes 138 in the furnace.

The system may also utilize a regulator 130. The fuel assist may be pressure or flow regulated, although not limited thereto. Using the regulator 130, fuel can be diverted to the hot surface igniter 104 through the side stream 112 to surround it or spray it. The fuel would cool the hot surface igniter 104 when it is not in service so it would not need to be retracted. In another embodiment, air or some other liquid could be used for cooling.

Because the hot surface igniter 104 may be a class 3 special type of igniter, no fuel train (e.g., separate fuel source) is required. Instead, the hot surface igniter 104 directly lights the main flame. The hot surface igniter 104 may take several seconds (e.g., 3-4) to heat up and ignite the fuel. This may be well within the any required time limit (e.g., 10 seconds) required for safe ignition using this class of igniter.

The fuel in the side stream 112 may extend the power input of the igniter (e.g., from 600 Joules for just electric to 150,000 joules with fuel assist). The hot surface igniter 104 may be shut off after a predetermined amount of time but the fuel assist may not be shut off but instead become part of the main fuel that is combusted at the burner end 108.

The hot surface igniter 104 may be arranged to help insure ignition, including isolating a portion of the fuel in the side stream 112 (e.g., fuel assist) so it can be heated above ignition temperature (and above main burner fuel temperature). In this way, the regulator 130 may provide for heating, although not limited thereto.

In order to determine when to activate/deactivate the hot surface igniter 104, a flame detector 140 may be employed. If the flame detector 140 detects a flame, then the hot surface igniter 104 can be deactivated. The control system is not shown since these control systems are common in the industry and known to those familiar with the art. Once the control system receives confirmation of the flame (or heat or some other indication of ignition), the hot surface igniter 104 may be deactivated. In another embodiment, the hot surface igniter 104 may be turned off after a predetermined amount of time (e.g., timer).

In the event that the burner flame is extinguished, the flame detector 140 may send a signal that will cause the control system to turn the hot surface igniter 104 on again under controlled conditions, and within seconds it will be hot enough to reignite the combustible fuel source. The control system may also control the fuel and air flow, although not limited thereto.

While the present teachings have been described above in terms of specific embodiments, it is to be understood that they are not limited to these disclosed embodiments. Many modifications and other embodiments will come to mind to those skilled in the art to which this pertains, and which are intended to be and are covered by this disclosure. It is intended that the scope of the present teachings should be determined by proper interpretation and construction of this disclosure and its legal equivalents, as understood by those of skill in the art relying upon the disclosure in this specification and the attached drawings. 

What is claimed is:
 1. A large-scale combustion system, comprising: a fuel line having a burner end; a burner at the burner end of the fuel line; a source of fuel flowing through the fuel line to the burner; a hot surface igniter adjacent to the fuel line; a flame holder disposed adjacent to the hot surface igniter; and a source of igniter combustion air; wherein the hot surface igniter is activated to ignite the source of fuel, the ignited fuel leaving the burner; and wherein the hot surface igniter is protected from a heat caused by the ignited fuel.
 2. The system of claim 1 wherein the hot surface igniter is protected from the heat by a side stream in the fuel line, the hot surface igniter disposed in the side stream, some of the source of fuel flowing through the side stream and cooling the hot surface igniter when deactivated.
 3. The system of claim 1 wherein the hot surface igniter is protected from the heat by a retractor retracting the hot surface igniter away from the heat when deactivated.
 4. The system of claim 1 further comprising a flame detector detecting ignition of the source of fuel at the burner, the hot surface igniter deactivated in response to detection of ignition.
 5. The system of claim 1 wherein the hot surface igniter is deactivated after a predetermined amount of time.
 6. The system of claim 1 further comprising a controller controlling the amount of fuel or air to the burner.
 7. The system of claim 1 wherein the source of fuel comprises natural gas.
 8. The system of claim 1 wherein the source of fuel comprises a liquid fuel.
 9. The system of claim 1 wherein the hot surface igniter is activated when no flame is detected at the burner.
 10. The system of claim 1 wherein the large-scale combustion system comprises an industrial boiler.
 11. The system of claim 1 wherein the large-scale combustion system comprises a power generator.
 12. A large-scale combustion system, comprising: a fuel line having a burner end; a burner at the burner end of the fuel line; a source of fuel flowing through the fuel line to the burner; an ignition device adjacent to the fuel line, the an ignition device activated to ignite the source of fuel, the ignited fuel leaving the burner; a side stream in the fuel line, the ignition device disposed in the side stream, some of the source of fuel flowing through the side stream and cooling the ignition device when deactivated; and a flame detector detecting ignition of the source of fuel at the burner, the ignition device deactivated in response to detection of ignition.
 13. The system of claim 12 wherein the ignition device comprises a hot surface igniter.
 14. The system of claim 12 wherein the ignition device comprises a direct spark type ignition device. 